not only do ar15 parts and ar10 parts not go together...there are two non-compatible ar10 formats as well (DPMS and Armalite). Your best bet is to buy a complete rifle or really think about what components you want and stick to either the DPMS format or the Armalite format.

Very true though I wouldn't refer to the "DPMS" standard, it's the original ArmaLite AR-10, Stoner or Knight's Armament SR-25 format. Magpul of course makes PMAGs for this format. DPMS came to the party much later. RRA uses a third format that uses FAL mags.

But things may be improving, as of early this year ArmaLite has returned to the original AR-10 magazine/upper format with their AR-10A. (Their AR-10B of course uses the M14 style mags.) I think we're beginning to see a standard forming around the SR-25/AR-10A and of course our military has a contract with KAC.

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The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
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US Army 1966-69, VFW Life Member, Retired Geek